Mainstream transducers transmitting and receiving ultra sound transmit and receive ultra sound, using piezoelectric effect and inverse piezoelectric effect of a ceramic piezoelectric element represented by PZT (lead zirconate titanate). This type of piezoelectric ceramic ultrasonic transducer currently still constitutes the majority of ultrasonic transducers in practical use. However, to take the place of this type, research and development of diaphragm type fine ultrasonic transducers having a structure in micrometer order by a semiconductor micro processing technology has been carried out since the 1990's (refer to Non-patent Document 1).
In a typical structure of such a transducer (an ultrasonic transducer 100p) is, as shown in a cross-sectional schematic view in FIG. 40, a capacitor is formed by a bottom electrode 2 (which is an electrode on a substrate side and will be herein after also referred to also merely as an electrode 2) and a top electrode 3 (which is an electrode on an outer diaphragm layer 5b side and will be herein after also referred to merely as an electrode 3) which are provided respectively on a substrate 1 and on a flat outer diaphragm layer 5b with a cavity 4 therebetween.
Hereinafter, for brevity of description, the direction where the ultrasonic transducer 100p receives ultra sound (downward direction in FIG. 40) will be referred to as z direction, the right direction in FIG. 40 will be referred to x direction, and the perpendicular downward direction with respect to the sheet of FIG. 40 will be referred to as y direction.
As shown in FIG. 40, when a voltage is applied between the electrodes 2 and 3, charges in opposite electric polarities are induced on the electrodes and attract each other, which displaces the outer diaphragm layer 5b. In this situation, when the outer surface of the outer diaphragm layer 5b is in contact with water or an organism, sound waves are radiated into such medium. This is the principal of electro-acoustic (ultra sound) conversion in transmission. On the other hand, when a DC bias voltage is applied to induce a certain amount of charges on the electrodes 2 and 3, and oscillation is forcibly applied to the diaphragm from a medium in contact with the diaphragm layer 5b, thereby displacing the diaphragm layer 5b, then a voltage is additionally generated between the electrodes 2 and 3 in accordance with the displacement. This principle of acoustic (ultra sound)-electro conversion in reception is the same as that of a DC bias type capacitor micro-phone adopted as a micro-phone for an audible band.
In order to form ultrasonic beams, a number of the above described transducers are disposed and arrayed, as shown in FIG. 43, to be used. In FIG. 43, plural hexagonal ultrasonic transducers 100 are electrically connected by connection 13 arranged between ultrasonic transducers to form a single channel partitioned by shown dashed lines 20. Ultrasonic pulses are transmitted and received, utilizing ultrasonic transducers. Herein, in imaging tomography of a subject from echo signals, the flatter the frequency spectrum of the electro-mechanical conversion efficiency of the ultrasonic transducers, the narrower the pulse width with respect to the time axis, achieving higher resolution. Further, the degree of freedom in controlling a device is advantageously increased, an example of which is that different frequencies can be selected, depending on the distance from an ultrasonic transducer to a subject. Therefore, a method is disclosed, in Patent Document 1, to achieve a broadband by simultaneously driving ultrasonic transducers 100, as shown in FIG. 44, having respective diaphragms in different diameters, the ultrasonic transducers being connected by connections therebetween to serve as a single element 14.
Further, Patent Document 2 discloses a capacitive ultrasonic transducer reinforced by a stiffing layer at the central portion of a film.
Still further, Patent Document 3 discloses an acoustic transducer with a structure, arranged above a cavity, having an insulating layer part and a top electrode which are disposed within the thickness dimension of a film.    Non-patent Document 1: “A surface micromachined electrostatic ultrasonic air transducer”, Proceedings of 1994 IEEE Ultrasonics Symposium, pp. 1241-1244    Patent document 1: the specification of U.S. Pat. No. 5,870,351    Patent document 2: the specification of U.S. Pat. No. 6,426,582    Patent document 3: the specification of U.S. Pat. No. 6,271,620